This invention relates to medical ultrasound diagnostic imaging, and in particular to systems and methods for providing more effective focusing of ultrasound waveforms.
In current ultrasound imaging systems, transducer probes which include many individual transducer elements are operated as phased arrays.
Delay profiles are applied either to transmit waveforms or to receive waveforms associated with individual transducer elements in order to achieve desired focusing characteristics. One prior-art approach is to use a simple delay profile in which all of the transducer elements of the transducer probe are focused at a single focal point. Another prior-art approach is to use a delay profile that provides a distributed focus, as for example the well-known Axicon profile that provides a line focus.
A third prior-art approach is to transmit two or more transmit focal zones simultaneously. This is typically done by superimposing two separate delay profiles such that each transducer element generates ultrasonic energy that focuses at each of the two or more focal zones. This approach is known as the multi-focus approach, and is described in U.S. Pat. Nos. 5,696,737, 5,675,554, 5,608,690, 5,740,128, as well as in U.S. patent application Ser. No. 09/089,463.
The Axicon focus is typically associated with large side lobe levels that can represent a substantial disadvantage in many clinical applications. The simultaneous transmission of multiple focal zones generally requires dedicated beamformer hardware. Also, if the probe is limited by regulatory power or thermal limits, then the use of the multi-focus approach may require reduced power which in turn is generally associated with a reduction in the signal to noise ratio.
Another approach for increasing depth to field includes the use of multiple sequential transmit events focused at respective ranges along with the same ultrasound line. This approach reduces the frame rate, though it can substantially increase the depth of field.
Thus, a need presently exists for an improved approach that increases depth of field while avoiding some or all of the disadvantages discussed above.